A brushless dc motor is a dc motor which dispenses with any brush and commutator but uses a plurality of magneto-sensitive elements for detection of the angular position of a magnetized rotor thereof. Hall effect elements are advantageously used as those magneto-sensitive elements.
Such a brushless dc motor is, for example, utilized for driving a magnetic recording disc in a magnetic disc recorder, which disc has usually a large mass and is to be rotated at a high speed. In such a case, rapid stopping of the brushless dc motor cannot be readily performed by merely ceasing the supply of electric power thereto. Thus, it is usual to employ frictional braking and/or dynamic braking in order to perform a quick stopping of the brushless dc motor especially when the brushless dc motor is subjected to a load with a large mass and/or a high rotational speed.
In case frictional braking is used, it is a problem that a brake shoe for the frictional braking has a relatively short life time and produces unwanted frictional heat. Furthermore, the frictional braking system is subject to leakage of brake oil which may adversely affect other elements around the braking system.
On the other hand, the dynamic braking does not produce a sufficiently large brake torque at a low rotational speed of the dc motor since the magnitude of the brake torque resulting from the dynamic braking is proportional to the rotational speed of the motor.
Another type of braking is known in the art, in which a reverse current is applied to the brushless dc motor so as to perform braking by reverse torque caused by the reverse current. However, a complicated sensing circuit is required for detecting the rotational direction of the motor so as to know the polarity of the reverse current to be applied to the motor.
It is accordingly a primary object of the present invention to provide an improved brushless dc motor drive and control circuit which can perform quick stoppage of the motor.
It is another object of the present invention to provide an improved brushless dc motor drive and control circuit which is simple in construction and accordingly economical.